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Radio Wave Propagation References

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Radio Wave Propagation References RADIO WAVE PROPAGATION REFERENCES “Almost Everything You Need to Know…”: Chapter 7: 53-68 “RAC Basic Study Guide 6th Ed:” 6.2, 6.3, 6.4, 6.5, 6.6, 6.8, 6.9, 6.10 “RAC Operating Manual 2nd Ed:” “The ARRL Handbook For Radio Amateurs 2001,78thEd:” Chapter 21: 1-37 "Radio Propagation" Wikipedia, The Free Encyclopedia. 6 Nov 2007 http://en.wikipedia.org/ “Chelmsford Amateur Radio Society” Intermediate Course (5) Antennas and Feeders TOC: •INTRO •RADIO WAVES •POLARIZATION •LINE OF SIGHT, GROUND & SKY WAVES •IONOSPHERE REGIONS •IONOSPHERIC LAYERS •PROPAGATION, HOPS, SKIPS ZONES •ABSORPTION AND FADING •SOLAR ACTIVITY AND SUN SPOTS •MF, HF CRITICAL FREQUENCIES •UHF, VHF, SPORADIC E, AURORAS, DUCTING •SCATTER, HF, VHF,UHF Major General Urquhart: “My communications are completely broken down. Do you •BEACONS really believe any of that can be helped by a cup of tea?” •SAMPLE QUESTIONS Corporal Hancock: “Couldn't hurt, sir” -Arnhem 1944 PROPAGATION - INTRO Propagation: How radio waves travel from point A to point B; and the events occurring in the transmission path that affect the communications between the points, stations, or operators. When the electrons in a conductor, (antenna wire) are made to oscillate back and forth, Electromagnetic Waves (EM waves) are produced. These waves radiate outwards from the source at the speed of light, 300 million meters per second. Light waves (waves we see) and radio waves (waves we hear)are both EM waves, differing only in frequency and wavelength. PROPAGATION – INTRO CONT’D EM waves travel in straight lines, unless acted upon by some outside force. They travel faster through a vacuum than through any other medium. As EM waves spread out from the point of origin, they decrease in strength in what is described as an "inverse square relationship". For example: a signal 2 km from its starting point will be only 1/4 as strong as that 1 km from the source. A signal 3 km from the source will be only 1/9 that at the 1 km point. HOWEVER….. Modern receivers are very sensitive and extremely small power provides usable signals. Waves can be received many thousands of kilometers from the transmitting station. For Example, Voyager 2 transmitted signals over many billions of kilometers from outer space with only 25 W of power! RADIO WAVES RADIO WAVES x Electric Field, E y Direction of z Propagation Magnetic Field, H • Electromagnetic radiation comprises both an Electric and a Magnetic Field. • The two fields are at right-angles to each other and the direction of propagation is at right-angles to both fields. • The Plane of the Electric Field defines the Polarisation of the wave. RADIO WAVES CONT’D Two types of waves: Transverse and Longitudinal Transverse waves: vibration is from side to side; that is, at right angles to the direction in which they travel A guitar string vibrates with transverse motion. EM waves are always transverse. RADIO WAVES CONT’D Longitudinal waves: Vibration is parallel to the direction of propagation. Sound and pressure waves are longitudinal and oscillate back and forth as vibrations are along or parallel to their direction of travel A wave in a "slinky" is a good visualization POLARIZATION • The polarization of an antenna is the orientation of the electric field with respect to the Earth's surface and is determined by the physical structure of the antenna and by its orientation • Radio waves from a vertical antenna will usually be vertically polarized. • Radio waves from a horizontal antenna are usually horizontally polarized. Direction of Propagation Direction of Propagation Vertically polarized omnidirectional Horizontally polarized directional dipole antenna yagi antenna RADIO WAVES CONT’D RADIO WAVES SPACE GROUND SKY REFLECTED DIRECT SURFACE LINE OF SIGHT, GROUND WAVE, SKY WAVE • Ground Wave is a Surface Wave that propagates or travels close to the surface of the Earth. • Line of Sight (Ground Wave or Direct Wave) is propagation of waves travelling in a straight line. These waves are deviated (reflected) by obstructions and cannot travel over the horizon or behind obstacles. Most common direct wave occurs with VHF modes and higher frequencies. At higher frequencies and in lower levels of the atmosphere, any obstruction between the transmitting antenna and the receiving antenna will block the signal, just like the light that the eye senses. • Space Waves: travel directly from an antenna to another without reflection on the ground. Occurs when both antennas are within line of sight of each another, distance is longer that line of sight because most space waves bend near the ground and follow practically a curved path. Antennas must display a very low angle of emission in order that all the power is radiated in direction of the horizon instead of escaping in the sky. A high gain and horizontally polarized antenna is thus highly recommended. • Sky Wave (Skip/ Hop/ Ionospheric Wave) is the propagation of radio waves bent (refracted) back to the Earth's surface by the ionosphere. HF radio communication (3 and 30 MHz) is a result of sky wave propagation. LINE OF SIGHT, GROUND WAVE, SKY WAVE KNOWLEDGE CHECK PROPAGATION, HOPS, SKIPS ZONES IONOSPHERE REGIONS • The ionosphere is the uppermost part of the atmosphere and is ionized by solar radiation. • Ionization is the conversion of atoms or molecules into an ion by light (heating up or charging) from the sun on the upper atmosphere. • Ionization also creates a horizontal set of stratum (layer) where each has a peak density and a definable width or profile that influences radio propagation. IONOSPHERE REGIONS THE IONOSPHERIC LAYERS The F layer: or region, is 120 km to 400 km above the surface of the Earth. It is the top most layer of the ionosphere. Here extreme ultraviolet (UV) (10-100 nm) solar radiation ionizes atomic oxygen (O). The F region is the most important part of the ionosphere in terms of HF communications. The F layer combines into one layer at night, and in the presence of sunlight (during daytime), it divides into two layers, the F1 and F2. The F layers are responsible for most skywave propagation of radio waves, and are thickest and most reflective of radio on the side of the Earth facing the sun. The E layer: is the middle layer, 90 km to 120 km above the surface of the Earth. This layer can only reflect radio waves having frequencies less than about 10 MHz. It has a negative effect on frequencies above 10 MHz due to its partial absorption of these waves. At night the E layer begins to disappear because the primary source of ionization is no longer present. The increase in the height of the E layer maximum increases the range to which radio waves can travel by reflection from the layer The D layer: is the innermost layer, 50 km to 90 km above the surface of the Earth. when the sun is active with 50 or more sunspots, During the night cosmic rays produce a residual amount of ionization as a result high-frequency (HF) radio waves aren't reflected by the D layer. The D layer is mainly responsible for absorption of HF radio waves, particularly at 10 MHz and below, with progressively smaller absorption as the frequency gets higher. The absorption is small at night and greatest about midday. The layer reduces greatly after sunset. A common example of the D layer in action is the disappearance of distant AM broadcast band stations in the daytime. THE IONOSPHERIC LAYERS CONT’D THE IONOSPHERIC LAYERS CONT’D Ionospheric Storms: Solar activity such as flares and coronal mass ejections produce large electromagnetic radiation incidents upon the earth and leads to disturbances of the ionosphere; changes the density distribution, electron content, and the ionospheric current system. These storms can also disrupt satellite communications and cause a loss of radio frequencies which would otherwise reflect off the ionosphere. Ionospheric storms can last typically for a day or so. D layer Absorption: Occurs when the ionosphere is strongly charged (daytime, summer, heavy solar activity) longer waves will be absorbed and never return to earth. You don't hear distant AM broadcast stations during the day. Shorter waves will be reflected and travel further. Absorption occurs in the D layer which is the lowest layer in the ionosphere. The intensity of this layer is increased as the sun climbs above the horizon and is greatest at noon. Radio waves below 3 or 4 MHz are absorbed by the D layer when it is present. When the ionosphere is weakly charged (night time, winter, low solar activity) longer waves will travel a considerable distance but shorter waves may pass through the ionosphere and escape into space. VHF waves pull this trick all the time, hence their short range and usefulness for communicating with satellites. Faraday rotation: EM waves passing through the ionosphere may have their polarizations changed to random directions (refraction) and propagate at different speeds. Since most radio waves are either vertically or horizonally polarized, it is difficult to predict what the polarization of the waves will be when they arrive at a receiver after reflection in the ionosphere. THE IONOSPHERIC LAYERS CONT’D SO!!!! • Solar radiation, acting on the different compositions of the atmosphere generates layers of ionization • Studies of the ionosphere have determined that there are at least four distinct layers of D, E, F1, and F2 layers. • The F layer is a single layer during the night and other periods of low ionization, during the day and periods of higher ionization it splits into two distinct layers, the F1 and F2. • There are no clearly defined boundaries between layers. These layers vary in density depending on the time of day, time of year, and the amount of solar (sun) activity.
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